Field of the Invention
The present invention relates to methods and apparatus for determining frequency response characteristics within an electric power grid.
Description of the Related Technology
The exchange of electrical power between providers and consumers takes place via an electricity distribution network or electric power grid. In such an electric power grid, electrical power is typically supplied by a combination of relatively large capacity power stations and relatively small capacity renewable energy sources.
Generators in large power stations, such as fossil fuel-burning or nuclear power stations, typically comprise rotating parts that have relatively high mass that are rotating at relatively high speeds, and accordingly are referred to as spinning generation. In the course of their normal operation, the spinning generators store relatively large amounts of kinetic energy. Smaller renewable energy sources, such as wind turbines and solar power generators store a much smaller amount of energy, or even no energy at all.
Typically, an electric power grid operates at a nominal grid frequency that is uniform throughout a synchronous area of the grid. For example, the UK mains supply nominally operates at 50 Hz. Grid operators are usually obliged to maintain the grid frequency to within predefined limits, for example the UK mains supply should be kept within 0.4% of the nominal 50 Hz grid frequency. If the balance between generation and consumption of electrical energy is not maintained (for example, if the total amount of generation cannot meet consumption during high demand periods, or if the output from a power generator changes, perhaps due to a fault in the generator) the net amount of energy stored in the generators of the grid can vary. This results in a change of the rotational speed of the spinning generators and a corresponding change in the operating frequency of the grid. Grid operators therefore use the system operating frequency as a measure of the balance between consumption and generation of electrical power in the grid.
A frequency response characteristic describes the response of the grid frequency to a change in balance between generation and consumption of electrical power in the grid. Examples of such frequency response characteristics include grid “stiffness” and grid “inertia”.
Grid stiffness is a property of the grid describing the extent (i.e. magnitude) of grid frequency response for a given power balance change. A synchronous electric power grid with a relatively high stiffness, for example, exhibits a relatively small change in grid frequency for a given power balance change. A stiff or strong grid typically has a low grid impedance and is typical of grids where the system generation capacity is large. Whilst stiffness is in general a static property of a given grid, it should be noted that in practice, for example in large grids, the generation and consumption capacity changes frequently, for example, when new providers are added to or removed from the grid and/or from intermittent sources of generation such as wind and solar. This means that in practice, grid stiffness can be a substantially dynamic property of a grid.
Grid inertia is a measure of the amount of energy stored in the electric power grid and influences the rate at which the operating frequency of the grid changes in response to a change in grid balance. Regions of a synchronous electric power grid that have a high proportion of spinning generation typically have a large amount of energy stored as rotational kinetic energy in the generators (that is they have high inertia) and therefore have a larger capacity to maintain the operating frequency of the grid at the nominal grid frequency. In contrast, regions of a synchronous electric power grid that have a low proportion of spinning generation have a relatively low amount of stored energy (that is they have low inertia) and therefore have less capacity to maintain the operating frequency of the grid at the nominal grid frequency. Consequently, the rate of change of frequency in high inertia regions of the grid is less than it is in low inertia regions of the grid, and the “inertia” may refer to this rate of change of frequency.
Since frequency response characteristics in the grid can provide an indication of how the grid will respond to sudden changes in consumption or generation, it is useful for grid operators to understand how grid frequency response characteristics vary across the electric power grid. Conventionally, grid frequency response characteristics are determined by using phasor instrumentation to make precise and high-resolution measurements of the grid operating frequency. Since such instrumentation is expensive, it is not practical to distribute widely; typically, measurements are made at a limited number of central nodes in a transmission grid. This means that the measurement is relatively insensitive to local variation within the grid.
Further, due to the large amounts of data that the measurements produce, the measurements are often analyzed off-line. This means that there is a delay in the determination of frequency response characteristics; this makes it difficult for network operators and the like to react in a timely manner to changes in frequency response characteristics.
It is an object of the present invention to at least mitigate some of the problems of the prior art.